1,678 research outputs found
Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry
The dissipation of energy flux in blazar jets plays a key role in the
acceleration of relativistic particles. Two possibilities are commonly
considered for the dissipation processes, magnetic reconnection -- possibly
triggered by instabilities in magnetically-dominated jets -- , or shocks -- for
weakly magnetized flows. We consider the polarimetric features expected for the
two scenarios analyzing the results of state-of-the-art simulations. For the
magnetic reconnection scenario we conclude, using results from global
relativistic MHD simulations, that the emission likely occurs in turbulent
regions with unstructured magnetic fields, although the simulations do not
allow us to draw firm conclusions. On the other hand, with local
particle-in-cell simulations we show that, for shocks with a magnetic field
geometry suitable for particle acceleration, the self-generated magnetic field
at the shock front is predominantly orthogonal to the shock normal and becomes
quasi-parallel downstream. Based on this result we develop a simplified model
to calculate the frequency-dependent degree of polarization, assuming that
high-energy particles are injected at the shock and cool downstream. We apply
our results to HBLs, blazars with the maximum of their synchrotron output at
UV-soft X-ray energies. While in the optical band the predicted degree of
polarization is low, in the X-ray emission it can ideally reach 50\%,
especially during active/flaring states. The comparison between measurements in
the optical and in the X-ray band made during active states (feasible with the
planned {\it IXPE} satellite) are expected to provide valuable constraints on
the dissipation and acceleration processes.Comment: 9 pages, 6 figures, accepted for publication by MNRA
Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds
By means of two- and three-dimensional particle-in-cell simulations, we
investigate the process of driven magnetic reconnection at the termination
shock of relativistic striped flows. In pulsar winds and in magnetar-powered
relativistic jets, the flow consists of stripes of alternating magnetic field
polarity, separated by current sheets of hot plasma. At the wind termination
shock, the flow compresses and the alternating fields annihilate by driven
magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the
wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated
flows), shock-driven reconnection transfers all the magnetic energy of
alternating fields to the particles, whose average Lorentz factor increases by
a factor of sigma with respect to the pre-shock value. In the limit
lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind,
the post-shock particle spectrum approaches a flat power-law tail with slope
around -1.5, populated by particles accelerated by the reconnection electric
field. The presence of a current-aligned "guide" magnetic field suppresses the
acceleration of particles only when the guide field is stronger than the
alternating component. Our findings place important constraints on the models
of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at
https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special
issue of Computational Science and Discovery on selected research from the
22nd International Conference on Numerical Simulation of Plasma
Happiness, housework and gender inequality in Europe
Although the last few decades have seen a progressive increase of gender equality in almost all dimensions of society, roles concerning childcare and domestic work remain highly gender-specific. Gender division of labour within the family varies considerably within and across countries. Gender systems are likely to have an important impact on individuals' well-being. Improved gender equality has enhanced the general well-being of women, although its extent may depend on the context in which women live and operate. This work considers the effect of the unequal division of labour within the household, between women and their partners, on women's own subjective assessment of happiness. We conducted the analysis using the European Social Survey data. We included 26 European countries and explored, exploiting a multi-level model to investigate the determinants of, women's differing levels of happiness across countries. In particular, we examined the extent to which gender equality at the country level can explain variation in happiness at the individual level. © The Author 2010. Published by Oxford University Press. All rights reserved
Particle Acceleration in Pulsar Wind Nebulae: PIC modelling
We discuss the role of particle-in-cell (PIC) simulations in unveiling the
origin of the emitting particles in PWNe. After describing the basics of the
PIC technique, we summarize its implications for the quiescent and the flaring
emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be
emerging that, in addition to the standard scenario of particle acceleration
via the Fermi process at the termination shock of the pulsar wind, magnetic
reconnection in the wind, at the termination shock and in the Nebula plays a
major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae"
edited by D. Torres for Springer, based on the invited contributions to the
workshop held in Sant Cugat (Barcelona), June 14-17, 201
Radio light curves during the passage of cloud G2 near Sgr A*
We calculate radio light curves produced by the bow shock that is likely to
form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*.
The shock acceleration of the radio-emitting electrons is captured
self-consistently by means of first-principles particle-in-cell simulations. We
show that the radio luminosity is expected to reach maximum in early 2013,
roughly a month after the bow shock crosses the orbit pericenter. We estimate
the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit
orientation and parameters. We show that the most promising frequencies for
radio observations are in the 0.1<nu<1 GHz range, for which the bow shock
emission will be much stronger than the intrinsic radio flux for all the models
considered.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Generation of near-equipartition magnetic fields in turbulent collisionless plasmas
The mechanisms that generate "seed" magnetic fields in our Universe and that
amplify them throughout cosmic time remain poorly understood. By means of
fully-kinetic particle-in-cell simulations of turbulent, initially unmagnetized
plasmas, we study the genesis of magnetic fields via the Weibel instability and
follow their dynamo growth up to near-equipartition levels. In the kinematic
stage of the dynamo, we find that the rms magnetic field strength grows
exponentially with rate , where
is the driving scale and is the rms turbulent velocity. In the
saturated stage, the magnetic field energy reaches about half of the turbulent
kinetic energy. Here, magnetic field growth is balanced by dissipation via
reconnection, as revealed by the appearance of plasmoid chains. At saturation,
the integral-scale wavenumber of the magnetic spectrum approaches . Our results show that turbulence -- induced by, e.g., the
gravitational build-up of galaxies and galaxy clusters -- can magnetize
collisionless plasmas with large-scale near-equipartition fields.Comment: 10 pages, 10 figures, PRL in pres
Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons
We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks
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